> 3.a. For qemu, I fear the produced binaries would conflict with the
> original ones. Is there something we can do in the build process to deal
> with that can of thing or sould we just expect the user deal with it on
> its own as Guix doest it very well on its own?

The specialized qemu should be an input to arcan, not a propagated input.
This way it won't conflict with anything.

You might have to configure or patch arcan so that it finds the specialized qemu
binaries in the input store folder.

> 3.b. The patched openal is used during build to produce a special binary
> (arcan_lwa which allows nested servers). It should normally be fetched
> during build time which is obviously not possible here. I suppose there
> is no clear answer here but how would you deal with that kind of
> behaviour? Create a modified openal package? Can we fetch multiple sources?

This happens a lot with other packages (e.g. the Go build system).  The answer
is simple: package the specialized OpenAL and provide it as input.
If arcan still insists on downloading openal, patch it so that it does not.

Hope that helps!

-- 
Pierre Neidhardt
https://ambrevar.xyz/